The present invention relates to a mold with a roughened surface which is used for casting metals, particularly aluminium and its alloys, by means of which mold the heat transfer, on first contact with the melt, is controlled in that the melt comes into contact only with the peaks on the roughened surface of the mold and an air gap is formed between the melt and the valleys or troughs in the roughened surface.
In continuous casting with moving molds the melt solidifies by coming directly into contact with the mold. Quality requirements make it necessary to control the transfer of heat accurately when the melt first makes contact with the mold. When the heat is extracted too quickly, as is the case with smoothly ground molds, there are often cold shuts in the cast product, which then leads to scrap. The transfer of a large amount of heat through the mold at the start also means high thermal stresses in the mold which can lead to cracks forming in the mold surface.
In the present state of the art there are two methods which are used to regulate the heat transfer between the melt and the mold. These are:
1. The surface of the mold is coated with a thermally insulating, protective layer.
2. The surface of the mold is roughened mechanically.
The use of insulating, protective layers often involves spraying a coat of lining material on the mold before casting commences. Ceramic layers which can be deposited by plasma spraying is another possibility. Experience has shown however that there are also disadvantages associated with the use of linings.
The lining must be deposited after each casting. It is especially important that the surface of the mold is coated uniformly, which depends of course on the skill of the operator. Nonuniform coating leads to areas in the cast strand or strip, where the rate of initial solidification differs. In most materials this leads to casting flaws which mostly appear in the form of surface porosity and surface cracks. Another problem is that there is always the danger of pick-up of particles from the coating material. For many products (e.g. foils) this leads to unacceptable contamination of the surface.
Experience has also shown that many aluminum alloys can be cast in continuously moving molds only if the initial solidification is sufficiently fast that the cell size at the surface of the cast strip is 10-20 .mu.m. The normal coatings however produce milder solidification conditions which then lead to surface flaws--surface porosity in particular.
Permanent ceramic layers have the disadvantage--in view of the high coating costs--that they exhibit only limited service lives. It is also difficult using this method of coating to achieve an initial solidification rate which is sufficiently fast for casting alloys.
In the case of a mechanically roughened mold the heat transfer is regulated by creating a suitably rough surface. When the melt comes into contact with a mold surface which, for example has been roughened by shot peening with steel balls then, if the metallostatic head is not too high, it comes into contact only with the peaks on the roughened surface, while an air cushion forms between the melt and the valleys on the roughened surface.
By appropriate dimensioning of the relative contact surface ##EQU1## where
F.sub.i =the contact surface of a peak on the surface
F.sub.o =the total mold surface area
n=the number of peaks on the surface and
by controlling the depth of roughness and the average spacing of neighboring peaks, the heat transfer through the mold can be regulated.
In the present state of the art there are two methods for mechanically roughening continuously moving molds:
(a) Grooves are created in the surface by means of chip forming processes (milling, planing). This method however exhibits various disadvantages. Because the demand for uniformity of heat transfer through the molds surface is very high, the demand for uniformity in the grooves is also very high. Modern machine tools can satisfy these requirements only for grooves spaced at about 1 mm or more apart. When the grooving is to be finer it is difficult to maintain uniform depth and uniform contact surface area. Furthermore, the machining costs increase markedly with increasing fineness of the grooves. Also, the surface to be machined in a continuous casting unit with moving molds is very large indeed--in a unit with moving, caterpillar track type molds, where the casting width is 2 m and the length 3 m, the mold surface area is about 30 m.sup.2.
Coarse grooving, i.e. a groove spacing of &gt;0.5 mm, leads to cracks, especially when casting wide strip, as too deep penetration of the metal in the valleys of the grooves results in rubbing between the solidified melt and the mold, to such an extent that the shrinkage on solidification is hindered.
(b) By striking the mold with hard particles--steel balls in particular--the surface is indented. This method leads to a uniform reduction in heat transfer which, at a suitable metallostatic pressure, permits the casting also of highly alloyed alloys (e.g. AlMg 4.5) with moving molds, in particular if the mold is made of copper. Practical experience has however revealed another disadvantage of this process which is described in the following:
During long production runs, it is unavoidable that impurities gather in the recesses formed by peening or otherwise impacting, and these decompose to produce gases when heated. These impurities include organic substances, hydroxides and various salts which contain water of crystallization. If, on casting, the metal comes into contact with such a contaminated area, then gas is produced. At high casting speeds in particular this gas is trapped between the melt and the mould as, because of the special feature of the roughening (craters adjacent to each other but separated by ridges) the flow of the gas parallel to the mould surface is greatly hindered as soon as the melt touches the surface. Bubbles of gas trapped between the mould and the solidifying metal, however, lead to flaws in the cast strip, which generally result in the strip being scrapped. It has also been found that the removal of these impurities by the various cleaning methods--taking into account the safety measures required in production--does not provide a suitable remedy.